1. Field of the Invention
The present invention relates to a loudspeaker structure and a method of assembling a loudspeaker, for a loudspeaker of the type that a magnetic circuit generating a repulsion magnetic field (hereinafter called "repulsion magnetic circuit") is generated by two magnets with the same polarity being faced with each other, a magnetic material plate is interposed therebetween, and a voice coil is placed in the magnetic field at the outer peripheral area of the plate.
2. Related Background Art
Various types of loudspeakers using a repulsion magnetic circuit have been proposed, for example, in Japanese Patent Laid-open Publication No. 56-34298, Japanese Utility Model Laid-open Publication No. 59-48197, Japanese Patent Laid-open Publication No. 60-3270, Japanese Utility Model Laid-open Publication No. 61-128896, and Japanese Patent Laid-open Publication No. 1-198400. The present inventors have proposed loudspeakers of this type having an improved performance.
In conventional methods of assembling a repulsion magnetic circuit, magnets 1 and a plate 2 are bonded and fixed together as shown in FIG. 8A, or magnets 1 and a plate 2 mounted on a holder 4 are fastened and fixed by a screw 12 as shown in FIG. 8B. In the case of a conventional loudspeaker proposed by the present inventors, a support shaft 4a is formed integrally with a holder 4 and holes of magnetic circuit components are fitted around the support shaft to fix them by using a screw or to bond and fix them, as shown in FIG. 8C.
In a repulsion magnetic circuit, two magnets with the same polarity being faced with each other are used. Therefore, during assembling them, the two repulsing magnets may jump out of a magnetic circuit holding unit. In order to avoid this, it is necessary to use strong adhesive agents and bond the components together. However, this bonding strength is not so much reliable in practical use.
In the case of the method illustrated in FIG. 8A, the lower magnet, plate, and upper magnet are bonded together in this order and disposed on the upper surface of the holder 4. However, each component has no structure for helping align its center. It is therefore difficult to assemble them.
In the screw fastening method illustrated in FIG. 8B, the length of the screw 12 is required to be a length corresponding to the total thickness of the two magnets 1 and plate 2 plus the length of an internal thread 4d. Using this long screw makes it difficult to align each magnetic circuit component at a high precision, and also poses some problem of the performance of a magnetic circuit itself. Furthermore, if strong magnets 1 or a thin plate 2 are used, a stronger repulsion force is generated. It becomes therefore difficult to assemble them by using a screw having a limited length.
In the screw fastening method illustrated in FIG. 8B, after inserting the screw 12 into the magnets 1 and plate 2, the head portion of the screw 12 is engaged with the internal thread 4d formed in the holder 4. Since the screw 12 is inserted into the magnetic circuit components, a worker cannot visually confirm the head portion of the screw 12 and the internal thread 4d, resulting in a low working efficiency. This structure is very disadvantageous particularly for an assembly process required to complete a work in several seconds.
Furthermore, center holes formed in the plate 2 and magnets 1 have some play relative to the screw 12. It is therefore very difficult to align the centers of the magnets 1 and plate 1. A poor center alignment generates an uneven distribution of magnetic fluxes supplied to a voice coil, causing a degraded sound quality.
In the magnetic circuit assembly methods illustrated in FIGS. 8B and 8C, the support shaft 4a or screw 12 is inserted into the magnet, plate, and magnet, in this order. The length of the support shaft 4a or screw 12 is limited and set to the length necessary only for fastening the magnetic circuit components. This length is short from the viewpoint of an assembly work. When the support shaft 4a or screw 12 is inserted first into the magnet and next into the plate 2 and when the plate 2 comes near the first magnet, it is attracted by the magnet and tightly contacts the magnet. This working condition is very bad. Furthermore, a danger in an assembly work may occur if fingers holding the plate are squeezed between the magnet and plate when the magnet attracts the plate.